rtai, xenomai: use the pre patch hook point

The post extract hook point is not really correct as what RTAI and
Xenomai extensions are doing is patching the kernel.

The post patch hook point doesn't work, because RTAI and Xenomai
patches would be applied *after* all other patches, while it sounds
more logical to apply them first, and *then* allow the user to apply
some platform/board specific patches if needed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>